1. Field of the Invention
This invention relates to an electronic endoscope in which provisions are made to correct for distortion and prevent shading.
2. Description of Related Art
In recent years, An endoscope whose elongated, insertable section is inserted in a narrow tubular cavity or human body to observe an organ has come into prominent use. In particular, an electronic endoscope is often used in which a solid-state image sensor, such as a CCD (charge coupled device), is placed at the distal end or ocular section of the endoscope so that observations can be made through a monitor.
In keeping with an increase of the number of pixels arrayed on the light-receiving surface of the CCD, a compact design of the CCD has recently been developed. Consequently, the problem is raised that off-axis chief rays of light passing through the objective lens system of the electronic endoscope are incident on pixels different from those on which they are naturally expected to be incident and proper colors are not reproduced. This phenomenon is called color shading.
FIG. 1 shows a positional relationship between a photosensor and a microlens in a conventional CCD. In this figure, reference symbols P1and P2represent chief rays emanating from an exit pupil 0 toward the CCD. Consider now the case where the chief ray P1is incident on one photosensor, situated close to the optical axis, of the CCD and the chief ray P2is incident on another photosensor, located farther from the optical axis, of the CCD. The chief ray P1passes through a microlens Am and enters a photosensor Bm located opposite thereto. The chief ray P2, however, after passing through a microlens Bn, fails to enter a photosensor Bn located opposite thereto. In this way, the photosensor Bn, located farther from the optical axis, of the CCD may give rise to the phenomenon that a marginal beam of light is eclipsed and thus an image to be formed becomes dark. This phenomenon is called luminance shading. In the following description, the term xe2x80x9cshadingxe2x80x9d refers to both the color shading and the luminance shading.
In the electronic endoscope, such shading is attributable to the fact that some of rays passing through the objective lens system are incident obliquely on the photosensors of the CCD.
Thus, in order to solve this problem, for example, Japanese Patent Preliminary Publication No. Hei 1-218286 proposes an apparatus in which a field lens is interposed between the objective lens system and the CCD so that rays, emerging from the objective lens system are incident almost perpendicularly on the CCD.
In general, the objective lens system used in the endoscope has a large angle of view so that a wide field can be observed, and hence is exceedingly subject to distortion.
Thus, an endoscope; objective proposed as an objective lens system designed to favorably correct for distortion is disclosed in Japanese Patent Preliminary Publication Sho 61-35414. This endoscope objective is composed of four lens units and has the feature that tile lens unit located closest, to the image side includes a meniscus lens.
Since the objective includes the meniscus lens, the entire length of the objective is increased accordingly. If the meniscus lens can be removed, the entire length of this lens system will be reduced. FIG. 2 shows an objective lens system constructed with three lens units for such a purpose. This objective lens system, however, encounters the difficulty that, in order to render emerging light incident normally on the CCD for the purpose of preventing shading, the diameter of the rear lens unit must be enlarged.
However, where the endoscope is used, notably for medicine, it is favorable to minimize the diameter of the insertable section and the length of a rigid portion at the distal end where the objective lens system is placed, in order to reduce pain caused to a patient when the endoscope is inserted and to facilitate the insertion thereof. The objective lens system which has the smallest possible diameter and the minimum length is suitable for use in the endoscope.
An objective lens system proposed to achieve this object is disclosed in Japanese Patent Preliminary Publication No. Hei 5-107470. This objective lens system is constructed with three lenses so that the field lens is removed to thereby diminish the diameter and entire length of the objective lens system and aberrations are favorably corrected.
However, the endoscope objective set forth in Sho 61-35414 mentioned above, although it is possible to correct for distortion, cannot render all beams of light emerging from this objective incident normally on the CCD, thus bringing about shading.
The objective lens system disclosed in Hei 5-107470 cannot prevent shading for a similar reason.
More recently, on the other hand, a CCD such as that disclosed in, for example, Japanese Patent Preliminary Publication No. Hei 5-346556 is proposed in which the positions of microlenses arranged opposite to respective photosensors are shifted closer to the optical axis than those of the photosensors, and thereby the production of shading is obviated. Where such a CCD is used, it is not necessary to make provision for preventing the shading on the objective side.
In this way, the CCD in which provision is made to prevent shading is most suitable for use in the endoscope, but no cases have as yet arisen in which this CCD has been used in the endoscope.
It is, therefore, a primary object of the present invention to provide an electronic endoscope in which distortion is favorably corrected and provision is made to prevent shading.
It is another object of the present invention to provide an electronic endoscope in which the compactness of the objective lens system is intended to minimize the diameter of the insertable section and the length of a rigid portion at the distal end of the endoscope.
In order to achieve the above objects, according to one aspect of the present invention, the electronic endoscope is equipped with an objective lens system in which chief rays directed toward the maximum image height are inclined outwardly with respect to the optical axis and a solid-state image sensor which has a light-receiving surface on which many pixels are arrayed and which provides a light beam inclined outwardly with respect to the optical axis and incident on the light-receiving surface with a higher power, in separating from the center of the light-receiving surface, than that incident normally on the light-receiving surface.
According to another aspect of the present invention, the electronic endoscope includes a plurality of objective lens systems, each of which is constructed so that chief rays directed toward the maximum image height are inclined outwardly with respect to the optical axis, and a solid-state image sensor which has a single light-receiving surface on which many pixels are arrayed so that images formed by the plurality of objective lens systems are received by this single light-receiving surface. The solid-state image sensor is such that, in separating from the center of each of the images formed on the single light-receiving surface by the plurality of objective lens systems, a light beam inclined outwardly with respect to the optical axis and incident on the single light-receiving surface is provided with a higher power than a light beam incident normally on the single light-receiving surface.
According to still another aspect of the present invention, the electronic endoscope is equipped with an objective lens system, illumination optical systems possessing such light-distribution characteristics that illuminance on the periphery of the visual field of the objective lens system is lower than that at the center thereof, and a solid-state image sensor in which, when light of uniform intensity is incident on the light-receiving surface thereof, each of pixels located on the periphery of the light-receiving surface produces a higher power than that located at the center thereof.
These and other objects as well as the features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.